I'm seriously thinking that Miriel didn't hallucinate at all. If you look at a couple of facts here and there regarding Sumia, it seemd she has some kind evil thoughts within her and the game's writing hints at it everywhere (I only played the NA version).
First off she suffers from depression and has very low self esteem, which might create an alternate personnality over time if not treated. Fighting a war and exhaustion from baking pies does not help. Also her conversations with MU implies that she can fake characters from novels, and most of them are grim. She even asks MU at some point if she's insane.
Also the Sumia ghost glitch where she appears out of nowhere in a scene if Chrom marries someone else is rather bizarre, sure it might just be a glitch but nintendo isn't the kind to keep stuff like that for no reason.
As for Myriel, it could be that fright and heat made her lose consciousness and Sumia, who didn't want her to find out, made her forget somehow and told her the friendship part. Maybe Cordelia can't approach Chrom for fear of Sumia, it would make some sense. I might be wrong though.